This invention relates to an improved cathode-ray tube having an internal coating comprised of carbon and a combination of silicates on the interior walls of the tube.
Cathode-ray tubes usually have a conductive internal coating on the interior walls of the tube. This coating is used to carry a high potential of the order of 15 to 30 kilovolts, which is applied at the anode button. The most common internal coating consists essentially of particulate graphite and an alkali silicate binder. The coating is applied to the walls of the tube by spraying and/or brushing as an aqueous coating composition, then dried, and then baked in air at about 400.degree. C. for 1 hour. After baking, this prior-art coating absorbs water vapor, carbon dioxide and possibly other gases from the ambient atmosphere. Subsequently, just prior to the tipping-off step, the tube is baked and exhausted of gases down to a pressure of about 10.sup.-.sup.6 torr, one purpose of which is to remove adsorbed gas from the internal coating. While this treatment is largely successful, very small amounts of adsorbed gases continue to be released during the operation of the tube. When these gases react with the cathode of the tube, the cathode becomes less efficient and finally inoperative to emit electrons. It is therefore desirable to reduce the amounts of gases released from the internal coating, at least to extend the life of the cathode.